1. Technical Field
The present invention relates to a solid electrolytic capacitor, wherein a laminated nonwoven fabric is used to form a separator.
2. Background Art
To enhance reliability and performance of electronic equipment, an electrolytic capacitor has been required that has longer life time and higher than normal electric characteristics compared with a conventional capacitor. For example, a capacitor that has excellent impedance characteristics in a high frequency region for electronic equipment has been required. Therefore, a solid electrolytic capacitor having low impedance in a high frequency region, using a solid electrolyte or a tetracyanoquinodimethane complex salt or the like having high electrical conduction, has been used. In addition, solid electrolytic capacitor is an electrical component having long life time and high reliability since an electrolytic solution is not used. Accordingly, applications of a solid electrolytic capacitor have been expanded because of capability of enhancing safety of electric appliances. On the other hand, as a solid electrolytic capacitor, a capacitor referred to as a winding type has been used in response to requirements for higher capacitance thereof. This winding type capacitor is typically formed by the following method. A cathode foil, an anode foil, and a separator are wound around, so that the separator is intervened between the cathode foil and the anode foil. After that, liquid such as a monomer (a monomer, a tetracyanoquinodimethane complex salt or the like, having high electrical conduction), an aqueous solution dispersed with fine particles of an electric conductive polymer, and an aqueous solution of the electric conductive polymer, is impregnated as a material of the electric conductive polymer, into this wound substance. Thus, a solid electrolyte layer is formed between electrodes by polymerization or binding of this electric conductive polymer material. The solid electrolytic capacitor formed in this way can be used as a capacitor element utilizing electrical conduction property. Conventionally, a separator which has been used in solid electrolytic capacitors has been a separator mainly composed of a cellulose component such as pulp or Manila hemp, as in an electrolytic solution-type capacitor.
However, the conventional separator has problems to be solved. That is, when a large quantity of a volatile substance, such as moisture, is contained inside the electronics parts that are supplied for surface mounting, the inside volatile substance evaporates all at once by heat, in the surface mounting step such as reflow soldering, which fractures parts and has thus increased fraction defective. Cellulose, which is a main component of pulp or Manila hemp composing the separator, contains a large quantity of moisture even in a usual state, and still more absorbed a large quantity of moisture or the like in the manufacturing step of a capacitor. That is, moisture entrained from the separator mainly composed of cellulose was a cause to increase fraction defective. Accordingly, to solve this demerit, in the conventional separator mainly composed of pulp, Manila hemp or the like, a capacitor component has been finished so as to reduce moisture content in the capacitor element, by a method of making a drying step excessive, to evaporate moisture or the like contained in cellulose such as pulp, or a method of carbonizing a cellulose component, in the manufacturing step of the capacitor. These methods required large scale equipment and thus a large quantity of energy in the steps, resulting in increase of cost of the solid electrolytic capacitor.
Still more, the above problems led to performance deterioration of a capacitor element itself. One of factors largely influencing performance of the solid electrolytic capacitor relates to necessity to form a dielectric oxide film layer as an anode (for example, in a solid aluminum electrolytic capacitor, an aluminum oxide layer) so as to attain a continuous layer and uniform thickness, at the whole surface of a metal foil having valve action, made of aluminum or the like. In the conventional separator, damage (damage by deterioration by heat, or contamination of the oxide film layer by gas components coming out from cellulose) generated in this oxide film layer in a heating step for evaporating moisture or the like, or a step of carbonizing cellulose or the like, resulting in deterioration of capacitor performance. For example, withstand voltage could not increase, or ripple current characteristics was poor, in the capacitor element. That is, in the solid electrolytic capacitor requiring high performance, because of necessity to adopt a heating step for this separator, performance of the capacitor cannot be enhanced and thus withstand voltage has been suppressed low. The solid electrolytic capacitor had a merit of longer life time as compared with an electrolytic solution-based capacitor, because of no worry of decrease in life time caused by leakage of an electrolytic solution, owing to no use of the electrolytic solution and thus small vaporization or leakage of an electrolyte. Irrespective of this merit, application range of the solid electrolytic capacitor is limited, because high withstand voltage similarly as an electrolytic solution capacitor was unattainable. Additionally, the solid electrolytic capacitor still had similar problems as in the electrolytic solution capacitor, that is, a problem that high capacitance of a capacitor was unattainable, and a problem that compact sizing of parts was unfeasible.
To solve these problems, various methods have been considered (refer to the PATENT LITERATURES 1 to 6). For example, in the PATENT LITERATURES 1 to 4, there has been described a separator having moisture suppressed, by using a PET or PET-type nonwoven fabric, as the separator.
In addition, in the PATENT LITERATURES 5 and 6, there has been attempted to form a uniform solid electrolyte layer using a nonwoven fabric made of vinylon fiber and/or a blend of vinylon fiber and synthetic fiber, aiming at the similar object as above. In addition, in a blended nonwoven fabric using vinylon fiber or the like, a binder is used in forming the nonwoven fabric. The binder is used for preventing a wound substance from not being produced stable, due to unraveling of fibers or extension of the nonwoven fabric in the winding step, when the separator is slit thinly.
As the other method, there has been attempted to use a woven fabric or a nonwoven fabric, using a glass fiber, as the separator.
In addition, as the other method, in the PATENT LITERATURE 8, there has been attempted to use a nonwoven fabric composed of a crystalline polymer (typically made by a melt blown method), as the separator.